The over-whelming sense of loss and waste can stir deeply passionate responses from almost anyone concerned with the health of our environment. Then, there's the smell of several thousand dead fish baking in the sun on a hot, steamy, August day -- that's something hard to forget.

The reactions are understandable. Beyond the obvious loss of the organisms, large-scale fish kills are indications of serious ecological disturbances. They are an apparent sign that conditions within the affected water body have deteriorated to such a degree that fish life is no longer supported. Answers are needed. What action is necessary to avoid a recurrence?

"Beyond the obvious loss of the organisms, large-scale fish kills are indications of serious ecological disturbances."

Fish kill determinations are a primary responsibility of DEQ's Water Quality Management Division field personnel based in eight regional offices across Louisiana. Deciding the cause of fish mortalities can be one of the most technically challenging and complex duties of the biologists, chemists, and other environmental specialists working in the DEQ field offices.

The investigation itself is much like detective work. It requires complete attention to detail, common sense, and a high degree of technical proficiency. Knowledge of fish behavior, scientific classification, and response to various pollutants is also a necessity.

"The cause of fish kills can be chemical, biological, or physical; more often, it is a combination of all these."

To solve the mystery, the investigator must use the available evidence and clues to piece together a series of events that often have long since passed. Proficiency in such a specialized area is best obtained through experience, so fish mortality determinations are reserved for the DEQ field scientists who have undergone a period of specific training on the subject.

Toxic chemicals are by no means the only cause of fish kills. Any discharge or sequence of events that interferes with or alters the quality of an aquatic system can trigger mass mortalities. The cause can be chemical, biological, or physical; more often, it is a combination of all of these.

A largemouth bass and a sunfish were among the casualities of this 1986 fish kill in Bayou Black near Houma. The fish mortalities were related to dissolved oxygen depletion in the water. Dense vegetation covering the water inhibited sunlight penetration, interfering with the production of oxygen by algae.

Diagnosing a fish kill begins at the time notification is received. Usually, the initial report comes by phone from a concerned citizen who has discovered dead or dying fish. All of the information provided is recorded. This includes such details as the location of the mortalities, when they were first noticed, and any unusual events witnessed immediately before the fish were discovered. The name and phone number of the caller are recorded in case questions arise later.

Once at the scene, the DEQ investigator must reduce the great number of possible causes of the fish kill to something more reasonable. Through proper observations, the most probable cause is placed in one of several broad categories. Is the problem oxygen-related, or are toxic substances involved? Are diseases implicated? Maybe there is a physical reason for the deaths.

Unfortunately, a conclusion cannot be reached through any single observation. Conclusions are based on several supporting observations, each of which serves to verify another.

The first step is simply to look at the fish. The species involved can provide good clues. For example, gar and the bowfin, or "choupique," have physical characteristics that enable them to use atmospheric oxygen. If oxygen concentrations within the water drop below the level needed to support most other species, gar and choupique can survive by just coming to the surface to gulp a little air.

Other groups of fishes such as the topminnows and killifishes have small, upturned mouths that enable them to take advantage of the water just below the surface. There is usually enough oxygen there to sustain them.

It follows, then, that if there are no gar, choupique, sailfin mollies, or mosquito fish among the carcasses, an oxygen-related problem begins to look like a possibility even before any water quality measurements are made.

In fresh water, oxygen-related mortalities usually include largemouth bass, various sunfishes, and shad, along with some of the larger carp and catfish that frequented the water body. In saltwater areas, mullet, red drum, sea trout, menhaden, and almost any other species expected to be in the area are commonly found among the dead fish when oxygen levels drop too low.

The size of the dead fish is also important. Typically, a fish kill caused by depleted oxygen supplies will not include the smaller fishes. Toxics-related fish kills, however, usually are characterized by the presence of all size classes, from juveniles to adults. In pesticide-related kills, for example, one- and two-inch sunfish are often observed; however, they do not usually appear in low-oxygen situations.

Leaving a blanket of dead fish in its wake, this kill involved almost extensively gulf menhaden. Such a single species incident in the open Gulf usually points to a cause not related to water quality.

The reason for this phenomenon is that younger, smaller-bodied fish require lesser amounts of oxygen than do larger fish. Consequently, the small fish can usually find enough oxygen. Bigger fish, however, have to extract much more oxygen from the water to sustain their larger body mass.

The aforementioned observations are made by a DEQ field investigator within the first few minutes after arriving at the scene of a fish kill. The next step is to measure directly certain key water-quality parameters. These include temperature, dissolved oxygen, pH, and conductivity. The measurements are made in the field with instruments that have been properly serviced and calibrated prior to each use.

Water testing is done at various "stations," or locations in the body of water. Choosing these locations is vital to the success of the diagnosis. Stations within the affected area are an obvious necessity, but it is important to make sure that areas outside of the mortalities are also checked. This helps to ensure that the area of poor water quality is completely delineated.

At each station, all testing is performed at multiple depths. Water quality can, and usually does, change considerably from top to bottom. In a healthy water body, for example, oxygen levels normally are slightly higher near the surface, where sunlight can penetrate to fuel the growth of algae.

It is possible to have too much of a good thing, however. Poorly treated domestic wastes, runoff from agricultural lands, and storm waters from over-fertilized yards can all provide the necessary ingredients to prompt an out-of-control growth, or "bloom," of algae. When this happens, oxygen levels at the surface often increase well beyond normal levels. Sophisticated instruments are not even needed to detect this condition, since the water color usually turns a definitive green.

Like other green plants, algae produce oxygen through photosynthesis while the sun is shining. But at night, or even during periods of heavy cloud cover, they begin to consume oxygen. Consequently, if there is an overpopulation of algae in the water, all of the dissolved oxygen in the water can be quickly and totally used up in one night, resulting in a fish kill.

In addition to oxygen-related and toxics-related causes, two other relatively common causes of fish kills are disease and physical reasons.

Large fish kills involving diseases or parasites usually occur when a population is under some environmental stress. Often there is an outward sign on the carcasses that indicates the presence of external or internal parasites. Also, many disease-related mortalities seem to involve a single species or a single group of species.

"Large fish kills involving diseases or parasites usually occur when a population is under some environmental stress."

The spread of fish diseases is an event of opportunity. That is, when conditions such as overpopulation, high water temperature, or low oxygen levels occur, fish become stressed. The disease-causing organisms can then take advantage of the opportunity to attack the fish's system while it is most vulnerable.

Other causes of fish mortalities are usually not related directly to water quality at all. Several years ago, large numbers of fish died in coastal Louisiana when water temperatures dropped to record lows. Fish are cold-blooded, and their body temperature is dependent on their surroundings. If they are caught in shallow water when the air temperature approaches freezing, entire schools of fish can perish.

In another unusual fish kill case near Ruston in 1986, DEQ field personnel spent hours of diagnostic work before it was finally determined that sunfish were dying from formic acid poisoning in a local lake because they were gorging themselves on fire ants driven from their nests by flood waters.

Once the cause of a fish kill is categorized, the investigation takes a decidedly more complex direction. The investigator must use all observations, laboratory data, and technical expertise to decide what is the most probable cause. If a discharge is implicated, enforcement actions are possible, provided that all rules of evidence have been followed meticulously.

If you should ever notice dead or dying fish, please call the nearest DEQ regional office as soon as possible. Transmitting all the right information to ensure a successful diagnosis will be easy, now that you know the science of how it's done.

Return to the Cover Stories Table of Contents Page

Return to the Louisiana Environmentalist Magazine Home Page